416ci LS3 Engine Build - Part 3 - Dyno Tuning

Hooking The Beast up to the Dyno

In last month's installment of our Building the Beast LS3 engine build series, we covered the assembly of the top end of our 416ci LS3, a process that involved bolting the RHS heads and FAST intake onto the previously assembled short-block. After a few days spent at Grimes Automotive Machine, watching Merlin James and Garry Scott Grimes machine and assemble the engine (and turning the odd wrench myself), the collection of high-end parts had finally become a single, tough-looking unit. The aluminum engine—crowned with the matte-gray intake, gleaming red fuel rails, and high-rise valve covers in crinkle black—looked nothing but purposeful. And since its purpose is to go fast, it was time to move it off the engine stand and onto the dyno, to see exactly how ill- tempered this beast could be.

By the time I got there, the motor was in the dyno cell, surrounded by the various black umbilici connecting it to the many inputs and outputs required to make it run: fuel lines, sensors for the dyno gauges, and wiring for the MSD ignition box and FAST XFI computer, along with a set of well-heat-cycled headers routed to the outside of the building. Garry Grimes sat at the controls, his left hand resting on the lever that controlled the throttle, the computer monitor above him filled with the brightly colored lines of a bar graph. John Lee, who was doing the actual tuning, sat on his right, leaning over his laptop and alternately scrolling through data-logging results and air/fuel tables.

The drill is simple: With the engine having previously been warmed to operating temperature, the Start switch is pressed until it comes to life and drops into its restless idle, provided courtesy of the 0.605/0.615 lift cam from Comp. Check the needles on all the gauges to be sure they're on the safe side of the middle; make sure all the inputs look right on the computer. Then, gently at first, Garry's hand begins smoothly pushing the lever forward, and that saucer-sized throttle body turns from a gold-colored disk to a flat line, and just for a moment as it hits wide-open throttle there's an expectant pause. Secretariat inhales. Then the engine's steady hammering changes tone as it sucks in air and goes from a snarl to a roar, the needle on the big, white tach hits the 6,800 rpm redline, and it's time to throttle it down and take a look at the charts.

Garry hands me a dyno sheet, and I can't help but scan the fine, dot-matrix type for the bottom number on the page, the bragging one. 595. As we continue to run the motor and Lee makes fine adjustments via his laptop, the peak horsepower continues to climb. It goes over six quickly. 625. Still climbing, and always hitting the peak around 6,700 rpm.

The volumetric efficiency (or VE) of a motor is basically a mechanical function determined by such things as cam, compression ratio, and other factors, and it can't really be changed by tuning. The VE table used in tuning is generally used as a fuel table: When you change its settings, you're effectively telling the engine computer that the engine is using more or less air, and the computer adjusts the fuel accordingly. This adjustment of air/fuel ratio is key in getting the engine to produce peak power while remaining in a safe range, so it doesn't run too rich (too much fuel) or too lean (too little fuel), either of which could cause damage.

To get there, the FAST XFI lets you read manifold pressure, spark timing, injector pulse, and other factors, as well as controlling such esoteric variables as injector phasing (similar to timing advance, only with the fuel injectors rather than the spark plugs). After the data is reviewed, changes are made to the calibration to adjust timing and fuel flow as required to optimize power.

Although we had originally planned to rely on the LS3's stock E38 computer, we eventually opted for the FAST XFI, in part because our stroker uses a cable-mounted throttle body, which is incompatible with the E38. The XFI's other advantages, however, are significant, making it a well-justified upgrade even without that consideration factored in.

Specifically, it gives the user an amazing amount of control over the engine, including everything from basic functions to ancillary items such as fan temperature and fuel-pump operation, torque-converter lockup, and controls for forced induction and nitrous oxide—with a provision for up to four stages of the latter.

Another unique feature that makes the XFI very appealing is its ease of tuning. In the event you need tuning help and no one suitable is available locally, you can connect your laptop to the XFI, establish an Internet connection, and allow another user to connect to your ECU, see what's going on, and change parameters as necessary.

As a matter of basic setup, the XFI can run the motor in several different modes, including speed density and a “load indexed” speed density that compensates for variations in barometric pressure. Ever driven in the mountains with a car tuned for the coast? That's the sort of problem load-indexed speed density is designed to avoid.

Another choice is that of bank versus sequential firing of the fuel injectors. In bank mode (also called batch-fire mode), a batch of four injectors—assuming an eight-cylinder engine—fire at the same time. Typically, each batch consists of the injectors for one cylinder bank. In sequential, each cylinder's injector fires once for each time the spark plug fires.

No matter which of these modes you select, the computer relies on feedback from a wide-band oxygen sensor mounted in the exhaust collector. This sensor allows for “closed loop” fuel-flow adjustment, whereby the computer makes fine alterations based on actual engine performance. While some systems use a narrow-band sensor that simply toggles between telling the computer “too high” or “too low,” a wide-band sensor assigns an actual numerical value, ranging between 9:1 and 16:1, to the air/fuel ratio.

An extension of this feature is the XFI's ability to run in Flex Fuel mode. Yes, with a little tuning and the addition of a Flex Fuel sensor, the XFI will adjust to accommodate different fuels, including E85, LP, compressed natural gas, and others, all of which function at a different ideal stoichiometric ratio than the 14.7:1 of pure gasoline. While I'm not wild about running a Corvette with any of these, in this world of ever-changing fuels, it sounds like cheap insurance to me.

Other tunable parameters include the way the computer offsets the injectors' opening based on fluctuating voltage from the battery (it used to have a table embedded in the computer's firmware; you can now adjust it on your own as well), after-start enrichment and decay (basically a choke-like effect), and rev limiting with both fuel and spark.

While the XFI can control all of these variables, doing so is not mandatory. The system offers three different levels of tuning—basic, intermediate, and advanced—so you can choose just how deep into the computer you want to go. No doubt, the roughly 200-page manual can seem a bit daunting, especially for those of us whose tuning experience consists of turning a screw on a Holley and listening until it “sounds right.” With that in mind, I left the initial tune to a professional, and if you're in the same boat I am, you'd probably be wise to do the same.

There are, however, other parts of the tuning process that I'll be tackling myself later on, such as configuring the outputs for the shift light and tach. (As a side note, while the tach signal from a standard LS computer is usually a four-cylinder output, the XFI input is eight-cylinder—something you'll need to know if you intend to use aftermarket gauges, as we'll be doing.) The most important part we'll tune, however, is the traction control, which is contained in the intermediate tuning mode.

The recurring question about putting this engine in my car is whether or not the end result will be controllable. (In the words of my girlfriend's father, "So…what car are you going to drive my daughter in?") Personally, the most fearsome thing I've driven is a five-speed-equipped '71 Stingray with a 550hp 502. The idea of adding about a hundred hp to that and then expecting to drive it with a manual transmission, on hills and in the rain, is more than a little intimidating. Hence my appreciation for the XFI's intelligent traction control, or ITC for short.

ITC measures (and subsequently limits) wheelspin by using a shaft-speed input, usually with a sensor that's attached to the driveshaft. While FAST offers a series of different sending units that use a pickup and a separate magnet mounted to the driveshaft, these are typically set up for a Ford 9-inch rear. In our case, we'll be using the digital speedometer output from our T56 Magnum transmission for this purpose. The Magnum comes with both mechanical and electronic outputs, and since we're keeping a cable-driven speedometer, we won't have to worry about splitting the signal between the speedo and traction control.

The computer reads the shaft-speed input—not for overall speed, but for change in speed, since sudden wheelspin will cause a sharp increase in driveshaft speed. The degree of driveshaft acceleration at which tire slippage occurs is among the variables you can input into the computer based on your particular setup and needs. Once the motor goes in the car, we'll be focusing pretty heavily on getting this right, to make sure the car is as streetable as reasonably possible.

And exactly how much power are we expecting to tame with ITC? Our best numbers came in at 635 hp at 6,700, with a peak torque of 543 lb-ft at 4,800 rpm. On the final dyno sheet (which doesn't read below 3,178 rpm), the torque comes on at 476, never really dropping much below that, and stays above 500 from 4,400 to 6,600, making for a pretty stout midrange and top end.

Once testing was done, the engine was put to bed in a crate and couriered over to Tray Walden's Street Shop in Alabama to meet the six-speed trans, custom fuel tank, and all the other bits and pieces required to shoehorn it between the rails of our '72 coupe project car, aka Scarlett.

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